EP4478592A1 - Power converter and method for operating a power converter - Google Patents

Abstract

For a power converter with a simple circuit design, with which the self-power supply, the balancing of the intermediate circuit and the pre-charging of the intermediate circuit from the AC side are possible, a DC/DC supply converter (16) is provided in the power converter (1), wherein a first DC converter connection (17) of the DC/DC supply converter (16) is connected to the DC self-supply bus (15) and a second DC converter connection (18) of the DC/DC supply converter (16) is connected to the DC voltage intermediate circuit (8) and during operation of the power converter (1), the intermediate circuit voltage (U _ZK ) or at least one capacitor voltage (U _C1 , U _C2 ) is applied to the second DC converter connection (18) of the DC/DC supply converter (16), and a DC/DC charging converter (20) is also provided in the power converter (1), wherein a first DC converter connection (22) of the DC/DC charging converter (20) is connected to the DC self-supply bus (15) and a second DC converter connection (21) of the DC/DC charging converter (20) is designed with a number of DC charging connections (21a, 21b) corresponding to the number of intermediate circuit capacitors (C _ZK ), wherein each DC charging connection (21a, 21b) is connected to one intermediate circuit capacitor (C _ZK ).

Description

The present invention relates to a power converter with a DC connection to which an electrical direct voltage can be applied during operation and an AC connection to which an electrical alternating voltage can be applied during operation, wherein a DC/AC main converter with a DC converter connection and an AC converter connection is provided in the power converter and the AC converter connection is connected to the AC connection of the power converter and the DC converter connection is connected to a DC intermediate circuit to which an intermediate circuit voltage is applied during operation of the power converter, wherein the DC intermediate circuit comprises at least two intermediate circuit capacitors connected in series and a capacitor voltage is applied to each of the intermediate circuit capacitors during operation of the power converter, wherein the DC connection of the power converter is connected to the DC intermediate circuit, wherein an AC/DC supply converter is provided in the power converter, which is connected to an AC converter connection of the AC/DC supply converter with the AC connection of the power converter and to a DC converter connection of the AC/DC supply converter is connected to a DC self-supply bus of the power converter. The invention also relates to a method for operating such a power converter.

Power converters are well-known power electronic devices that work as inverters (DC/AC converters) to convert a direct current (DC) (electrical current or voltage) into an alternating current (AC) (electrical current or voltage), or vice versa as rectifiers (AC/DC converters). Bidirectional power converters are also known, which allow energy to flow in both directions. A direct current is therefore present at one connection of the power converter and an alternating voltage is present at the other connection.

The DC voltage quantity is provided by a DC voltage source such as a battery, photovoltaic system, etc., or is absorbed by a DC voltage sink such as a battery, inverter, etc. The AC voltage quantity is absorbed by an AC voltage sink such as an AC grid, etc., or is provided by an AC voltage source such as an AC grid, wind turbine, etc.

A DC intermediate circuit consisting of one or more intermediate circuit capacitors connected in series is usually provided between the DC part and the AC part of the converter. A DC/AC converter, which is at least unidirectional, is connected to the DC intermediate circuit, which converts the intermediate circuit voltage into an AC voltage of the converter, or converts an AC voltage of the converter into an intermediate circuit voltage.

As a power electronic device, a power converter comprises at least one semiconductor switch, such as a transistor, usually a plurality of semiconductor switches, which are controlled by a control unit of the power converter to convert energy from DC to AC, or vice versa. This results in certain requirements for the power converter.

The control unit is usually a microprocessor-based component or an integrated circuit (such as an application-specific integrated circuit (ASIC) or a field programmable gate array (FPGA)) that requires an electrical power supply. The power converter therefore requires its own electrical power supply, for example for operating the control unit or for cooling the power converter or for other peripherals. The power converter includes its own power supply power supply for the electrical power supply, which can be supplied on the AC side, for example from an AC network (AC voltage source or AC voltage sink) or an AC output of the power converter. An example of such a power supply for the power converter can be the WO 2013/178546 A1 An AC-side supply is particularly necessary when no DC voltage source is available, for example at night with a PV system or when the battery is empty. If a DC voltage source is available, a DC-side energy supply can also be provided, for example by means of an inverter (DC/DC converter) as a self-power supply power supply. A combination of an AC-side supply with a DC-side supply is also known, for example from DE 10 2008 032 317 A1 In particular, it is also known to provide a small AC power supply for the standby mode of the power converter with a more powerful DC power supply for feeding power from the DC voltage source into an AC voltage sink for the purpose of reducing losses.

The DC intermediate circuit of the converter is often designed as a split intermediate circuit with at least two intermediate circuit capacitors in series. A DC voltage is applied to each intermediate circuit capacitor, which together make up the intermediate circuit voltage. With two intermediate circuit capacitors, the intermediate circuit voltage at the intermediate circuit capacitors would be halved if the same intermediate circuit capacitors were used. However, as is well known, asymmetrical DC voltages can occur at the intermediate circuit capacitors during operation of the converter for various reasons. Such asymmetries can lead to improper operation of the converter or to damage to parts of the converter. When the converter is in operation, the DC voltages at the intermediate circuit capacitors must therefore be balanced. Various approaches are known for this. In a simple design, balancing resistors are simply connected in parallel to the intermediate circuit capacitors. However, current flows through the balancing resistors during operation. permanent electrical current, which leads to losses and a reduced efficiency of the converter. There are also known approaches that use the division of the intermediate circuit into several intermediate circuit capacitors to supply the converter with its own energy. However, this requires switching converters connected to the intermediate circuit capacitors. Examples of this are the EP 2 826 126 B1 be taken.

A further balancing circuit can be EP 2 760 103 A1 It describes a balancing circuit that consists of a transformer whose primary side is connected to the intermediate circuit voltage via a switching element. The secondary side of the transformer consists of two secondary windings, each connected in series with a diode. Each secondary winding with a diode is connected in parallel with an intermediate circuit capacitor. The diodes ensure that the intermediate circuit capacitor is charged with the lower voltage via the transformer. However, balancing cannot be ensured by charging via the transformer alone, because this can also overload an intermediate circuit capacitor (charge beyond a nominal capacitor voltage).

When the converter is switched off, the intermediate circuit is usually also discharged. In order to put the converter into operation, the intermediate circuit must therefore first be precharged. If no DC voltage is available on the DC side, the intermediate circuit must be precharged on the AC side, for example from an AC network. An example of an AC-side precharging of the intermediate circuit capacitors is the WO 2019/206910 A1 to be taken.

However, these requirements - self-power supply, balancing and pre-charging of the intermediate circuit from the AC side - require a wide variety of circuits that should be implemented in a power converter with as little loss as possible. This makes a power converter, apart from the power electronics parts for energy conversion, complex in terms of circuitry because a number of additional circuits are necessary.

It is therefore an object of the present invention to provide a power converter with a simple circuit design, with which the self-power supply, the balancing of the intermediate circuit and the pre-charging of the intermediate circuit from the AC side are possible.

This object is achieved with a power converter mentioned at the outset with the features of claim 1, as well as with a method mentioned at the outset for operating such a power converter according to claim 5.

By providing an additional DC/DC charging converter that is fed from the DC bus, the various operating modes can be easily implemented in combination with the DC/DC supply converter, so that in particular the balancing of the intermediate circuit and the pre-charging of the intermediate circuit from the AC side can be easily implemented. The self-power supply of a control unit can also be ensured if required by providing the AC/DC and DC/DC supply converters.

In particular, balancing is carried out automatically and in an energy-efficient manner by recharging the intermediate circuit capacitors via the DC/DC charging converter from the DC self-supply bus and simultaneously discharging the DC voltage intermediate circuit via the DC/DC supply converter into the DC self-supply bus. The energy taken from the intermediate circuit capacitors is at least partially used to recharge the intermediate circuit capacitors until symmetry is restored.

The DC self-supply bus, to which the DC supply voltage is applied during operation, can also ensure the self-supply of certain components of the converter. If at least one control unit with a supply input is provided in the converter, with the supply input being connected to the DC self-supply bus, then the self-supply of the control unit can also be ensured.

In an advantageous embodiment, at least one DC/DC main converter with a first DC converter connection of the DC/DC main converter and a second DC converter connection of the DC/DC main converter is provided between the DC connection and the DC voltage intermediate circuit, wherein the DC connection of the power converter is connected to the DC voltage intermediate circuit via the at least one DC/DC main converter in that the first DC converter connection of the DC/DC main converter is connected to the DC connection of the power converter and the second DC converter connection of the DC/DC main converter is connected to the DC voltage intermediate circuit. This allows different DC sources or DC sinks to be connected, wherein in the case of several DC/DC main converters, several DC sources or DC sinks can also be connected.

In an advantageous embodiment, the DC/DC charging converter comprises a transformer with a primary winding on a transformer core, wherein the primary winding is connected to the DC self-supply bus via at least one clock switching element, that on the secondary side of the transformer on the transformer core a plurality of secondary windings corresponding to the number of intermediate circuit capacitors are provided, wherein a turns ratio between the primary winding and the respective Secondary winding is selected so that when the converter is operating, a DC charging voltage is induced on a secondary winding that essentially corresponds to the capacitor voltage required on the associated intermediate circuit capacitor, and that a diode is connected in series with each secondary winding so that when the converter is operating, the diode conducts if the capacitor voltage is lower than the respective DC charging voltage. With this circuit, no active control is required on the secondary side of the transformer, since the diodes regulate the energy flow independently.

With the power converter according to the invention, different operating modes can be realized in a simple manner.

The DC/DC supply converter connected to the DC link can be switched off to precharge the DC link capacitors via the AC/DC supply converter, the DC self-supply bus and the DC/DC charging converter in the absence of DC voltage at the DC connection.

To symmetrize the DC intermediate circuit via the DC/DC supply converter, the DC self-supply bus and the DC/DC charging converter when a DC voltage is present at the DC connection, the DC/DC supply converter can be switched on so that the intermediate circuit voltage or at least a capacitor voltage of an intermediate circuit capacitor of the DC intermediate circuit is converted into the DC supply voltage via the DC/DC supply converter and fed into the DC self-supply bus. The DC intermediate circuit is thus discharged into the DC self-supply bus via the DC/DC supply converter and recharged from the DC self-supply bus via the DC/DC charging converter in order to establish the symmetry of the capacitor voltages. This symmetrization can also be used for pre-charging.

In order to prevent the intermediate circuit capacitors of the DC link from being overloaded in terms of voltage, both during pre-charging and during balancing, the capacitor voltages can be recorded and the output power of the DC/DC charging converter reduced if one of the capacitor voltages is greater than a specified nominal voltage. This reduces the charging current for the intermediate circuit capacitors and prevents an overvoltage on the intermediate circuit capacitors.

• Fig.1 einen erfindungsgemäßen Stromrichter,
• Fig.2 eine weitere Ausgestaltung eines erfindungsgemäßen Stromrichters und
• Fig.3 eine vorteilhafte Ausführung eines DC/DC-Ladewandlers des Stromrichters.

The present invention is described below with reference to the Figures 1 to 3 which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

• Fig.1 a power converter according to the invention,
• Fig.2 a further embodiment of a power converter according to the invention and
• Fig.3 an advantageous design of a DC/DC charging converter of the power converter.

Fig.1 shows a power converter 1 according to the invention (in the form of a voltage intermediate circuit converter) with a DC connection 2 and an AC connection 3. An electrical direct voltage DCG, such as a DC voltage or a DC current, can be applied to the DC connection 2 during operation of the power converter 1 via two connection contacts DC+, DC. A direct voltage source that provides the direct voltage DCG or a direct voltage sink that receives the direct voltage DCG can be connected to the DC connection 2. An alternating voltage ACG can be applied to the AC connection 3 during operation of the power converter 1. An alternating voltage source that provides the alternating voltage ACG or an alternating voltage sink that receives the alternating voltage ACG can be connected to the AC connection 3. The alternating voltage ACG can be single-phase or multi-phase, for example three-phase as in the embodiment according to Fig.1 . At least two connection contacts (not shown) are therefore provided on the AC connection 3.

On the AC side, in front of the AC connection 3, a separating unit 4, such as a separating relay, can also be provided in the power converter 1 in order to electrically separate the AC connection 3 of the power converter 1, or an AC voltage source connected to it during operation, from the other parts of the power converter 1. Such a design is shown in Fig.2 shown.

Likewise, further circuit components can be provided in the converter 1 on the AC side, preferably in front of a separating unit 4, such as a known electrical output filter, which generally consists of a connection of capacitors and chokes. However, since such circuit components are irrelevant to the invention and are also sufficiently known, they are not shown in the figures.

A DC/AC main converter 5 is provided on the AC side of the power converter 1. The DC/AC main converter 5 is used to transfer energy to the power converter 1 and, when the power converter 1 is operating, converts a direct voltage value into an alternating voltage value that is present at an AC converter connection 6 of the DC/AC main converter 5, or vice versa in the case of a bidirectional first DC/AC main converter 5 (in the opposite case, the DC/AC main converter 5 works as an AC/DC converter). The AC converter connection 6 of the DC/AC main converter 5 is connected to the AC connection 3 of the power converter 1, if necessary via a separating unit 4 and/or other circuit components of the power converter 1.

The AC energy transmission side of the power converter 1 is thus formed by the AC part of the DC/AC main converter 5 and the AC connection 3, and if necessary the separation unit 4 and/or further circuit components between the separation unit 4 and the DC/AC main converter 5.

The DC converter connection 7 of the DC/AC main converter 5 is connected to a DC intermediate circuit 8. The DC intermediate circuit 8 consists of at least two intermediate circuit capacitors C _ZK , which preferably have the same capacitance values. The at least two intermediate circuit capacitors C _ZK are connected in series with one another. When the power converter 1 is operating, an intermediate circuit voltage U _ZK is applied to the DC intermediate circuit 8, which is divided into capacitor voltages U _C1 , U _C2 , each of which is applied to an intermediate circuit capacitor C _ZK due to the serial connection of the intermediate circuit capacitors C _ZK . The DC/AC main converter 5 thus receives the intermediate circuit voltage U _ZK as a DC voltage quantity for conversion into the AC voltage quantity at the AC converter connection 6.

In a possible embodiment of the power converter 1, as in the embodiment according to Fig.1 , when the converter 1 is in operation, the intermediate circuit voltage U _ZK corresponds to the DC voltage DCG present at the DC connection 2, in this case a DC voltage. In another possible version of the converter 1, as in Fig.2 As shown, at least one at least unidirectional DC/DC main converter 9 is arranged between the DC connection 2 and the DC voltage intermediate circuit 8, which is used to transmit energy to the power converter 1 when the power converter 1 is in operation and converts a DC voltage DCG, in this case a DC voltage, present at the DC connection 2 into the intermediate circuit voltage U _ZK , or vice versa in the case of a bidirectional converter. For example, a PV power converter can have several PV strings, each connected to a DC/DC main converter 9. Or several batteries could also be provided, each connected to a DC/DC main converter 9.

The DC energy transmission side of the power converter 1 is thus formed by the DC intermediate circuit 8 and the DC part of the DC/AC main converter 5, or by the DC/DC main converter 9, the DC intermediate circuit 8 and the DC part of the DC/AC main converter 5.

During operation, the energy transfer of the power converter 1 takes place from the DC connection 2 via the DC energy transfer side and the AC energy transfer side to the AC connection 3, or vice versa in the case of a bidirectional converter.

For the operation of the power converter 1, a control unit 10 is provided which controls the individual parts of the power converter 1 via control lines S1, S3, in particular the DC/AC main converter 5 via a control line S1 and possibly also the at least one DC/DC main converter 9 via a control line S3. If present, the control unit 10 can also control the isolating unit 4 via a control line S2. The type of control signals sent via the control lines S1, S2, S3 is irrelevant for the invention. During operation, the control unit 10 controls in the DC/AC main converter 5, and possibly also in the DC/DC main converter 9, in a known manner, in particular switching elements such as semiconductor switches such as transistors.

The control unit 10 is preferably designed as a microprocessor-based component or as an integrated circuit, such as an application-specific integrated circuit (ASIC) or a field programmable gate array (FPGA), which requires an electrical power supply for operation.

For operation, the power converter 1 therefore requires its own energy supply 11, which in particular provides the electrical energy for operating the control unit 10 and, if necessary, also for other basic supply components or peripherals. The power converter 1's own energy supply 11 comprises an AC/DC supply converter 12, whose AC converter connection 13 is connected to the AC connection 3 of the power converter 1. This also ensures, in the case of a separation unit 4, that a voltage supply via the AC voltage source or AC voltage sink connected to the power converter 1 is possible even when the separation unit 4 is open. The DC converter connection 14 of the AC/DC supply converter 12 is connected to a DC self-supply bus 15 of the power converter 1. The internal energy supply 11 of the power converter 1 also comprises a DC/DC supply converter 16, the first DC converter connection 17 of which is connected to the DC internal supply bus 15 and the second DC converter connection 18 of which is connected to the DC intermediate circuit 8, so that during operation of the power converter 1 in this embodiment according to Fig.1 the intermediate circuit voltage U _ZK is present at the second DC converter connection 18 of the DC/DC supply converter 16. The control unit 10 has a supply connection 19 which is connected to the DC internal supply bus 15. However, a separate power supply unit could also be provided for the electrical energy supply of the control unit 10. In this case, the supply connection 19 would be connected to the power supply unit or switchably to the power supply unit or the DC internal supply bus 15.

This means that the DC self-supply bus 15 can be supplied with a DC supply voltage Uv from the AC/DC supply converter 12 via the AC side of the power converter 1, as well as via the DC/DC supply converter 16 and the DC voltage intermediate circuit 8. The power supply of the control unit 10 can thus be provided during operation of the power converter 1 from both the AC side and the DC side of the power converter 1. The AC/DC supply converter 12 and/or the DC/DC supply converter 16 can also be controlled by the control unit 10 via a control line S4, S5. However, it can also be provided that the AC/DC supply converter 12 is always active and therefore no control line S5 is required (as in Fig.2 ).

When the power converter 1 is in operation, the DC supply voltage Uv is provided, for example, via the AC/DC supply converter 12 and the AC connection 3 of the power converter 1. If required, the DC internal supply bus 15 can be additionally or alternatively supplied with the DC supply voltage Uv via the DC/DC supply converter 16. However, according to the invention, the DC/DC supply converter 16 has another task in addition to the optional voltage supply of the DC internal supply bus 15, as explained below.

A DC/DC charging converter 20 is also provided in the power converter 1. A first DC converter connection 22 of the DC/DC charging converter 20 is connected to the DC internal supply bus 15, so that the DC supply voltage Uv is present at this first DC converter connection 22 during operation. At the second DC converter connection 21 of the DC/DC charging converter 20, a plurality of DC charging voltages U _L1 , U _L2 are provided at DC charging connections 21a, 21b of the DC converter connection 21 of the DC/DC charging converter 20 in accordance with the number of intermediate circuit capacitors C _ZK . In the embodiment according to Fig.1 thus two DC charging voltages U _L1 , U _L2 are provided. The DC/DC charging converter 20 thus converts the DC supply voltage Uv of the DC self-supply bus 15 into the plurality of DC charging voltages U _L1 , U _L2 . The DC/DC charging converter 20 therefore has a DC converter connection 22 with a plurality of DC charging connections 21a, 21b, wherein each of the DC charging connections 21a, 21b is connected to one of the intermediate circuit capacitors C _ZK . During operation of the power converter 1, the DC/DC charging converter 20 can generate a DC charging voltage U _L1 , U _L2 at the DC charging connections 21a, 21b at least as required, which is applied to the respectively connected intermediate circuit capacitor C _ZK .

Although it should be noted that a low DC voltage, for example in the range of 10-50V, preferably 20V or 24V, is applied to the DC internal supply bus 15 as the DC supply voltage Uv, while the intermediate circuit voltage U _ZK is generally in the range of a few hundred volts to a thousand volts. The DC internal supply bus 15 is not used in particular for the energy transmission of the power converter 1.

This circuit with AC/DC supply converter 12, DC/DC supply converter 16 and DC/DC charging converter 20 enables the self-power supply of the power converter 1 (as already explained above), but also the balancing of the DC intermediate circuit 8 and also the pre-charging of the DC intermediate circuit 8 from the AC side in a simple and energy-efficient manner, as explained below.

Precharging the DC intermediate circuit 8 from the AC side of the power converter 1 is necessary, for example, if no DC voltage DCG is available at the DC connection 2 of the power converter 1 (for example at night in the case of a PV system at the DC connection 2) and the DC intermediate circuit 8 cannot therefore be fed via the DC connection 2 or the DC/DC main converter 9. In this case, the AC/DC supply converter 12 feeds the DC self-supply bus 15 via the AC connection 3, to which an AC voltage ACG is present, for example from an AC network. To precharge the DC intermediate circuit 8, the DC supply voltage Uv of the DC internal supply bus 15 is converted by the DC/DC charging converter 20 into the DC charging voltages U _L1 , U _L2 , which are applied to the intermediate circuit capacitors C _ZK , whereby the intermediate circuit capacitors C _ZK are charged to the DC charging voltages U _L1 , U _L2 . During the precharging of the DC intermediate circuit 8 via the DC/DC charging converter 20, the DC/DC supply converter 16, which is also connected to the DC intermediate circuit 8 and the DC internal supply bus 15, is switched off in order to prevent the DC/DC supply converter 16 from simultaneously drawing energy from the DC intermediate circuit 8 and discharging the DC intermediate circuit 8 again, which would contradict precharging.

However, in certain configurations, the pre-charging function of the power converter 1 enables another operating mode of the power converter 1. For example, if a PV system is connected to the DC connection 2, the PV system cannot provide a direct voltage DCG at night. This means that the direct voltage intermediate circuit 8 is discharged. However, the power converter 1 is also connected to an AC network at night via the AC connection 3. However, the direct voltage intermediate circuit 8 can be pre-charged via the AC/DC supply converter 12 and the DC/DC charging converter 20, and the power converter 1 can therefore basically be put into operation. When the power converter 1 is in operation, the power converter 1 could, for example, generate reactive power that is fed into the AC network. The ability of a power converter 1 for PV applications to deliver reactive power is a requirement of the network operators in many countries. The energy for generating the reactive power could be taken from the AC grid by the DC/AC main converter 5, for example as active power, and fed into the DC voltage intermediate circuit 8, specifically into the intermediate circuit capacitors C _ZK .

With the power converter 1 according to the invention, a reactive power feed into an AC network could thus be carried out via the AC/DC supply converter 12, the DC self-supply bus 15 and the DC/DC charging converter 20 even when no DC voltage is present at the DC connection 2.

If an asymmetry of the capacitor voltages U _C1 , U _C2 at the intermediate circuit capacitors C _ZK occurs, for example due to component values of the intermediate circuit capacitors C _ZK not being exactly the same, such an asymmetry can be compensated according to the invention as follows.

If one of the intermediate circuit capacitors C _ZK is not charged enough (for example one capacitor voltage U _C1 , U _C2 at nominal voltage and another below it), for example due to different capacitance values of the intermediate circuit capacitors C _ZK , the DC charging voltage U _L1 , U _L2 provided by the DC/DC charging converter 20 ensures that the intermediate circuit capacitor C _ZK that is not charged enough is recharged with a capacitor voltage U _C1 , U _C2 that is less than the associated DC charging voltage U _L1 , U _L2 . The electrical output power provided by the DC/DC charging converter 20 thus flows into the direct voltage intermediate circuit 8 and ensures that the intermediate circuit capacitor C _ZK that is not charged enough is recharged. At the same time, however, the DC intermediate circuit 8 is also discharged via the DC/DC supply converter 16, with all intermediate circuit capacitors C _ZK being discharged together, since its second DC converter connection 18 is connected to the DC intermediate circuit 8. The DC/DC supply converter 16 is therefore active at the same time as the DC/DC charging converter 20. The discharged electrical energy is not lost, but is made available to the DC/DC charging converter 20 for recharging via the DC internal supply bus 15. The electrical energy is thus guided in a circuit. It can therefore also be the case that energy that was taken from an intermediate circuit capacitor C _ZK is fed back into it. This (ideally) ensures that both intermediate circuit capacitors C _ZK are charged to the nominal voltage (e.g. 500V). In other words, this recharging and simultaneous discharging can also be referred to as balancing the intermediate circuit capacitors C _ZK .

If only the intermediate circuit capacitor C _ZK were to be recharged, the undercharged intermediate circuit capacitor C _ZK would be charged to the same level as the other intermediate circuit capacitor C _ZK , which may, however, be above the nominal voltage and is therefore not desired. If only a discharge If this were to happen, both intermediate circuit capacitors C _ZK would be reduced by the same level and the asymmetry would remain. However, by simultaneously recharging and discharging, the capacitor voltage U _C1 , U _C2 can be balanced.

When the power converter 1 is in operation, the control unit 10 can coordinate the recharging and discharging, i.e. the balancing. Measured values of the capacitor voltages U _C1 , U _C2 , which the DC/AC main converter 5 can provide, can serve as a basis for this. The DC/DC supply converter 16 and the DC/DC charging converter 20 can be controlled by the control unit 10 via the control lines S4, S6 to the DC/DC charging converter 20 and DC/DC supply converter 16.

Another example of symmetrization is when the capacitor voltage U _C1 , U _C2 are different, but both are smaller than the nominal voltage, i.e., the voltages are divided differently due to component tolerances, for example. In this case, too, symmetrization is carried out as described above via the DC/DC supply converter 16, the DC internal supply bus 15 and the DC/DC charging converter 20. The DC/DC supply converter 16 is connected on the input side via the DC converter connection 18 to the DC intermediate circuit 8, i.e. to the intermediate circuit voltage U _ZK . The DC/DC supply converter 16, which feeds the DC internal supply bus 15, draws electrical power from the DC intermediate circuit 8, whereby the DC/DC supply converter 16 discharges the intermediate circuit capacitors C _ZK and reduces the capacitor voltages U _C1 , U _C2 on all intermediate circuit capacitors C _ZK . Since this extracted electrical power flows into the internal supply of the power converter 1, this is very energy efficient. If one of the capacitor voltages U _C1 , U _C2 drops below the assigned DC charging voltage U _L1 , U _L2 of the DC/DC charging converter 20, the DC/DC charging converter 20 ensures symmetrization again by recharging as described above. The electrical energy of the overcharged intermediate circuit capacitor C _ZK is thus circulated during balancing via the DC/DC supply converter 16, the DC self-supply bus 15 and the DC/DC charging converter 20 and is supplied to the now undercharged intermediate circuit capacitor C _ZK .

Since the DC/DC supply converter 16 is also responsible for its own energy supply, it is usually designed to be more powerful than the charging converter 20, i.e. with a higher nominal output power. For example, the DC/DC charging converter 20 could be designed with an output of 20W and the DC/DC supply converter 16 with an output of 50W.

It is obvious that the voltage levels of the various voltages of the various converters, in particular the DC/DC supply converter 16 and the DC/DC charging converter 20, are related to each other and to the nominal intermediate circuit voltage U _ZK or the nominal capacitor voltages U _C1 , U _C2 are to be coordinated. The nominal voltages are the voltages that should be present when the converter 1 is operating properly. Such voltage-related dimensioning of the converters can easily be carried out by a specialist.

It is useful, for example, if the achievable DC charging voltages U _L1 , U _L2 of the DC/DC charging converter 20 each correspond at least to the nominal capacitor voltages U _C1 , U _C2 desired at the intermediate circuit capacitors C _ZK , whereby the sum of the DC charging voltages U _L1 , U _L2 advantageously corresponds to the nominal intermediate circuit voltage U _ZK .

In order to ensure symmetrization in a simple manner, it could be provided that the output voltage of the DC/DC supply converter 16, which is fed into the DC internal supply bus 15 as the DC supply voltage Uv, is greater than the output voltage of the AC/DC supply converter 12, which is also fed into the DC internal supply bus 15. For example, the DC/DC supply converter 16 could provide an output voltage of 24V and the AC/DC supply converter 12 an output voltage of 20V. The DC internal supply bus 15 is thus primarily fed by the DC/DC supply converter 16 when the power converter 1 is operating and the symmetrization described above is ensured. Of course, it could also be provided that the control unit 10 controls the DC/DC supply converter 16 and the AC/DC supply converter 12 in order to enable symmetrization.

A voltage overload (more than the nominal voltage of an intermediate circuit capacitor C _ZK ) of each individual intermediate circuit capacitor C _ZK should usually be prevented. This can also be seen as asymmetry in the case of differently charged intermediate circuit capacitors C _ZK and can be prevented by balancing via the DC/DC supply converter 16, the DC self-supply bus 15 and the DC/DC charging converter 20, because the DC/DC supply converter 16 lowers the capacitor voltage U _C1 , U _C2 at each intermediate circuit capacitor C _ZK and uses it via the DC self-supply bus 15 and the DC/DC charging converter 20 to compensate for the asymmetry. It can also be provided that the capacitor voltages U _C1 , U _C2 are recorded by the control unit 10. If at least one of the intermediate circuit capacitors C _ZK reaches its specified nominal capacitor voltage, provision can be made to reduce the electrical output power of the DC/DC charging converter 20, for example by the control unit 10, so that a voltage overload of the intermediate circuit capacitors C _ZK by the DC/DC charging converter 20 is avoided.

In order to prevent a voltage overload of the intermediate circuit capacitor C _ZK at symmetrical capacitor voltage U _C1 , U _C2 , it can also be provided that the capacitor voltages U _C1 , U _C2 are detected by the control unit 10. If at least one of the intermediate circuit capacitors C _ZK reaches its nominal capacitor voltage, it can be provided to reduce the electrical output power of the DC/DC charging converter 20, for example by the control unit 10, so that a voltage overload of the intermediate circuit capacitors C _ZK is avoided.

An asymmetry of the capacitor voltages U _C1 , U _C2 can also occur when precharging the DC voltage intermediate circuit 8, as described above. This means that even when precharging via the DC/DC charging converter 20, balancing can take place via the DC/DC supply converter 16, the DC internal supply bus 15 and the DC/DC charging converter 20 as described above. This means that the DC/DC supply converter 16 can also be activated during precharging to balance. This operating case therefore represents balancing as described above. During precharging, the power consumed by the DC/DC supply converter 16 can also be limited, for example by the control unit 10, in order not to hinder precharging, but still to reduce the capacitor voltages U _C1 , U _C2 if necessary.

Likewise, during pre-charging, it can be provided that the capacitor voltages U _C1 , U _C2 are detected by the control unit 10. If at least one of the intermediate circuit capacitors C _ZK reaches its nominal voltage, it can be provided to reduce the electrical output power of the DC/DC charging converter 20, for example by the control unit 10, so that a voltage overload of the intermediate circuit capacitors C _ZK is avoided. This can take place with or without switching on the DC/DC supply converter 16.

During pre-charging, for example in preparation for reactive power feed-in, the AC/DC supply converter 12 is active because it supplies the DC self-supply bus 15. All of this can be controlled via the control unit 10, for example via the corresponding control lines.

Fig.2 shows a power converter 1 according to the invention in operation with a direct voltage source 30, in this case a PV system, which is connected to the DC connection 2 and provides the direct voltage DCG. In the embodiment of the power converter 1 according to Fig.2 a DC/DC main converter 9 is also provided, as already described above. An AC voltage sink 31, in this case a three-phase AC network, is connected to the AC connection 3. The power converter 1 enables an energy flow from the DC voltage source 30 to the AC voltage sink 31, for example a feed-in operation of the PV system into the AC network. However, it goes without saying that the DC voltage source 30 could also be another voltage source, such as a battery, a DC network, etc. In In this case, the energy flow could also be reversed and electrical energy could also be transferred from the AC side to the DC side of the power converter 1. For example, the power converter 1 could be intended to charge a battery connected to the DC connection 2 from an AC network connected to the AC connection 3 or a wind turbine. With such a configuration, bidirectional operation is also conceivable, with energy flowing from the battery or a DC network into an AC network or vice versa from the AC network into a battery or a DC network. A combination of unidirectional and bidirectional is also possible, since several DC/DC main converters 9 are possible.

In the cases referred to in the Fig.1 and 2 In the power converters 1 described, the DC/DC supply converter 16 is connected to the intermediate circuit voltage U _ZK of the DC voltage intermediate circuit 8 via the DC converter connection 18.

However, it would also be conceivable in principle for the DC converter connection 18 of the DC/DC supply converter 16 to be connected to one of the capacitor voltages U _C1 , U _C2 or to several capacitor voltages U _C1 , U _C2 . In this case, the DC/DC supply converter 16 also has at least one middle connection, depending on the number of connected capacitor voltages U _C1 , U _C2 . It can therefore be operated in such a way that a more heavily charged intermediate circuit capacitor C _ZK is discharged. Accordingly, energy that was taken from one intermediate circuit capacitor C _ZK can essentially be fed directly to another intermediate circuit capacitor C _ZK . The DC/DC supply converter 16 involved in the balancing of the intermediate circuit capacitors C _ZK thus accelerates the duration of the balancing.

For the invention, it is irrelevant how the individual converters of the power converter 1 are designed. In principle, all possible and known circuit topologies for the DC/AC main converter 5, the DC/DC main converter(s) 9 (if present), the AC/DC supply converter 12, the DC/DC supply converter 16 and the DC/DC charging converter 20 are conceivable and can be used.

Fig.3 shows a particularly advantageous embodiment of a DC/DC charging converter 20. In this embodiment, the DC/DC charging converter 20 comprises a transformer 35 with a primary winding 36 on a transformer core 39, which is connected to the DC internal supply bus 15 via a clock switching element 37. The clock switching element 37 can be controlled by the control unit 10, for example, via a control line S6 in order to clock the supply voltage Uv of the DC internal supply bus 15 on the primary winding 36 with a predetermined clock frequency. On the secondary side of the transformer 35, a plurality of secondary windings 38a, 38b corresponding to the number of intermediate circuit capacitors C _ZK are provided on the same transformer core 39, in Example of the Fig.3 two secondary windings 38a, 38b. The turns ratio between the primary winding 36 and the respective secondary winding 38a, 38b is preferably selected such that the DC charging voltage U _L1 , U _L2 induced on a secondary winding 38a, 38b during operation essentially corresponds (except for a forward voltage of the diode D1, D2) to the capacitor voltage U _C1 , U _C2 desired on the respectively assigned intermediate circuit capacitor C _ZK . A diode D1, D2 is connected in series with each secondary winding 38a, 38b in such a way that the diode D1, D2 conducts when the capacitor voltage U _C1 , U _C2 is smaller than the respective DC charging voltages U _L1 , U _L2 . A diode D1, D2 blocks when a capacitor voltage U _C1 , U _C2 is equal to or greater than the respective DC charging voltages U _L1 , U _L2 and conducts when the capacitor voltage U _C1 , U _C2 is less than the respective DC charging voltages U _L1 , U _L2 by the breakdown voltage of the diode D1, D2. This circuit of the DC/DC charging converter 20 thus ensures automatic recharging until the capacitor voltages U _C1 , U _C2 of the intermediate circuit capacitors C _ZK are balanced when the intermediate circuit capacitors C _ZK are undercharged, as described above. The energy transmitted by the DC/DC charging converter 20 can also be controlled via the clocking of the clock switching element 37 or a suitable operating management strategy (for example, a set peak current) in the control unit 10 in order, for example, to influence how quickly an asymmetry at the intermediate circuit capacitors C _ZK is compensated.

In the illustrated embodiments, one control unit 10 is shown in each case. However, it goes without saying that a distributed control with a plurality of individual control units 10 can also be provided in the power converter 1. In a distributed control, at least one control unit 10, preferably all control units 10, can be electrically supplied via the internal energy supply 11 as described above.

Claims (8)

Power converter with a DC connection (2) to which an electrical direct voltage quantity (DCG) can be applied during operation, and an AC connection (3) to which an electrical alternating voltage quantity (ACG) can be applied during operation, wherein a DC/AC main converter (5) with a DC converter connection (7) and an AC converter connection (6) is provided in the power converter (1), and the AC converter connection (6) is connected to the AC connection (3) of the power converter (1), and the DC converter connection (7) is connected to a DC intermediate circuit (8) to which an intermediate circuit voltage (U _ZK ) is applied during operation of the power converter (1), wherein the DC intermediate circuit (8) comprises at least two intermediate circuit capacitors (C _ZK ) connected in series, and a capacitor voltage (U _C1 , U _C2 ) is applied to each of the intermediate circuit capacitors (C _ZK ) during operation of the power converter (1), wherein the DC connection (2) of the power converter (1) is connected to the DC voltage intermediate circuit (8), wherein an AC/DC supply converter (12) is provided in the power converter (1), which is connected to an AC converter connection (13) of the AC/DC supply converter (12) to the AC connection (3) of the power converter (1) and is connected to a DC self-supply bus (15) of the power converter (1) by a DC converter connection (14) of the AC/DC supply converter (12), characterized in that a DC/DC supply converter (16) is provided in the power converter (1), wherein a first DC converter connection (17) of the DC/DC supply converter (16) is connected to the DC self-supply bus (15) and a second DC converter connection (18) of the DC/DC supply converter (16) is connected to the DC voltage intermediate circuit (8) and, during operation of the power converter (1), the intermediate circuit voltage (U _ZK ) or at least one capacitor voltage (U _C1 , U _C2 ) is present at the second DC converter connection (18) of the DC/DC supply converter (16), and that a DC/DC charging converter (20) is provided in the power converter (1), wherein a first DC converter connection (22) of the DC/DC charging converter (20) is connected to the DC self-supply bus (15) and a second DC converter connection (21) of the DC/DC charging converter (20) is designed with a number of DC charging connections (21a, 21b) corresponding to the number of intermediate circuit capacitors (C _ZK ), wherein each DC charging connection (21a, 21b) is connected to one intermediate circuit capacitor (C _ZK ). Power converter according to claim 1, characterized in that between the DC connection (2) and the DC voltage intermediate circuit (8) at least one DC/DC main converter (9) with a first DC converter connection (25) of the DC/DC main converter (9) and a second DC converter connection (26) of the DC/DC main converter (9) is provided, wherein the DC connection (2) of the power converter (1) is connected via the at least one DC/DC main converter (9) is connected to the DC voltage intermediate circuit (8) by connecting the first DC converter connection (25) of the DC/DC main converter (9) to the DC connection (2) of the power converter (1) and connecting the second DC converter connection (26) of the DC/DC main converter (9) to the DC voltage intermediate circuit (8). Power converter according to claim 1 or 2, characterized in that the DC/DC charging converter (20) comprises a transformer (35) with a primary winding (36) on a transformer core (39), wherein the primary winding (36) is connected to the DC self-supply bus (15) via at least one clock switching element (37), that on the secondary side of the transformer (35) on the transformer core (39) a number of secondary windings (38a, 38b) corresponding to the number of intermediate circuit capacitors (C _ZK ) are provided, wherein a turns ratio between the primary winding (36) and the respective secondary winding (38a, 38b) is selected such that during operation of the power converter (1) a DC charging voltage (U _L1 , U _L2 ) is induced on a secondary winding (38a, _38b ), which substantially corresponds to the capacitor voltage (U _C1 , U _C2 ) and each DC charging voltage (U _L1 , U _L2 ) is applied to an intermediate circuit capacitor (C _ZK ), and that a diode (D1, D2) is connected in series with each secondary winding (38a, 38b) so that during operation of the power converter (1) the diode (D1, D2) conducts when the capacitor voltage (U _C1 , U _C2 ) is smaller than the respective DC charging voltage (U _L1 , U _L2 ). Power converter according to one of claims 1 to 3, characterized in that at least one control unit (10) with a supply input (19) is provided in the power converter (1), wherein the supply input (19) is connected to the DC self-supply bus (15). Method for operating a power converter (1) with a DC connection (2) to which an electrical direct voltage quantity (DCG) is applied, and with an AC connection (3) to which an electrical alternating voltage quantity (ACG) is applied, wherein in the power converter (1) an intermediate circuit voltage (U _ZK ) of a direct voltage intermediate circuit (8) with at least two intermediate circuit capacitors (C _ZK ) connected in series, each of which has a capacitor voltage (U _C1 , U _C2 ) applied, is converted into the alternating voltage quantity (ACG) by means of a DC/AC main converter (5), or vice versa, wherein the intermediate circuit voltage (U _ZK ) of the direct voltage intermediate circuit (8) is provided in normal operation via the DC connection (2), or vice versa, wherein a DC self-supply bus (15) of the power converter (1) is connected to a DC supply voltage (Uv), characterized in that a DC/DC charging converter (20) converts the DC supply voltage (Uv) of the DC self-supply bus (15) into one of the number of intermediate circuit capacitors (C _ZK ) corresponding number of DC charging voltages (U _L1 , U _L2 ) and each DC charging voltage (U _L1 , U _L2 ) is applied to an intermediate circuit capacitor (C _ZK ) and that a DC/DC supply converter (16) connected to the DC voltage intermediate circuit (8) is switched off in order to precharge the intermediate circuit capacitors (C _ZK ) of the DC voltage intermediate circuit (8) via the AC/DC supply converter (12), the DC self-supply bus (15) and the DC/DC charging converter (20) if there is no DC voltage (DCG) at the DC connection (2) or the DC/DC supply converter (16) is switched on so that the intermediate circuit voltage (U _ZK ) or at least one capacitor voltage (U _C1 , U _C2 ) of an intermediate circuit capacitor (C _ZK ) of the DC voltage intermediate circuit (8) is converted into the DC supply voltage (Uv) via the DC/DC supply converter (16) and fed into the DC self-supply bus (15) in order to symmetrize the intermediate circuit capacitors (C _ZK ) of the DC voltage intermediate circuit (8) via the DC/DC supply converter (16), the DC self-supply bus (15) and the DC/DC charging converter (20) when a DC voltage quantity (DCG) is applied to the DC connection (2). Method according to claim 5, characterized in that during precharging and/or during balancing the capacitor voltages (U _C1 , U _C2 ) are detected and an output power of the DC/DC charging converter (20) is reduced if one of the capacitor voltages (U _C1 , U _C2 ) becomes greater than a predetermined nominal capacitor voltage. Method according to claim 5 or 6, characterized in that the direct voltage quantity (DCG) at the DC connection (2) is converted by a DC/DC main converter (9) of the power converter (1) into the intermediate circuit voltage (U _ZK ) of the direct voltage intermediate circuit (8), or vice versa. Method according to one of claims 5 to 7, characterized in that a control unit (10) of the power converter (1) is supplied with the DC supply voltage (Uv) via the DC self-supply bus (15).

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